In electronic controlling apparatuses that are used in electric power steering apparatuses, etc., because large currents or high voltages are supplied to a power circuit that is mounted to a circuit board, the power circuit generates heat, and since this generated heat can lead to malfunction and failure of electronic components, it is desirable to radiate the heat generated in the power circuit effectively.
Thus, in conventional electronic control units in which semiconductor modules that constitute a power circuit are mounted to a circuit board by connecting metal sheets and terminals thereof to a wiring pattern that is formed on a first surface of the circuit board, heat generated in the semiconductor modules has been radiated by disposing a radiator so as to be connected to the wiring pattern by means of a thermally conductive member to transfer the heat generated in the semiconductor modules to the radiator by means of the wiring pattern and the thermally conductive member (see Patent Literature 1, for example).
However, in conventional electronic control units, because electronic components such as the semiconductor modules can only be mounted to the first surface of the circuit board, packing density of the parts is low.
In consideration of such conditions, conventional motor driving circuit boards have been proposed in which a laminated circuit portion is constituted by alternately laminating conductor layers and resin insulating layers, a metal sheet is disposed so as to contact the lowermost insulating layer, a heat-dissipating via is disposed so as to contact the uppermost insulating layer and the lowermost insulating layer on which electronic components are mounted, and heat generated in the electronic components is radiated to the metal sheet by means of the heat-dissipating via and the lowermost insulating layer (see Patent Literature 2, for example).